rfc1812.txt
`
`Network Working Group F. Baker, Editor
`Request for Comments: 1812 Cisco Systems
`Obsoletes: 1716, 1009 June 1995
`Category: Standards Track
`
` Requirements for IP Version 4 Routers
`
`Status of this Memo
`
` This document specifies an Internet standards track protocol for the
` Internet community, and requests discussion and suggestions for
` improvements. Please refer to the current edition of the "Internet
` Official Protocol Standards" (STD 1) for the standardization state
` and status of this protocol. Distribution of this memo is unlimited.
`
`PREFACE
`
` This document is an updated version of RFC 1716, the historical
` Router Requirements document. That RFC preserved the significant
` work that went into the working group, but failed to adequately
` describe current technology for the IESG to consider it a current
` standard.
`
` The current editor had been asked to bring the document up to date,
` so that it is useful as a procurement specification and a guide to
` implementors. In this, he stands squarely on the shoulders of those
` who have gone before him, and depends largely on expert contributors
` for text. Any credit is theirs; the errors are his.
`
` The content and form of this document are due, in large part, to the
` working group's chair, and document's original editor and author:
` Philip Almquist. It is also largely due to the efforts of its
` previous editor, Frank Kastenholz. Without their efforts, this
` document would not exist.
`
`Table of Contents
`
` 1. INTRODUCTION ........................................ 6
` 1.1 Reading this Document .............................. 8
` 1.1.1 Organization ..................................... 8
` 1.1.2 Requirements ..................................... 9
` 1.1.3 Compliance ....................................... 10
` 1.2 Relationships to Other Standards ................... 11
` 1.3 General Considerations ............................. 12
` 1.3.1 Continuing Internet Evolution .................... 12
` 1.3.2 Robustness Principle ............................. 13
` 1.3.3 Error Logging .................................... 14
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` 1.3.4 Configuration .................................... 14
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`rfc1812.txt
` 1.4 Algorithms ......................................... 16
` 2. INTERNET ARCHITECTURE ............................... 16
` 2.1 Introduction ....................................... 16
` 2.2 Elements of the Architecture ....................... 17
` 2.2.1 Protocol Layering ................................ 17
` 2.2.2 Networks ......................................... 19
` 2.2.3 Routers .......................................... 20
` 2.2.4 Autonomous Systems ............................... 21
` 2.2.5 Addressing Architecture .......................... 21
` 2.2.5.1 Classical IP Addressing Architecture ........... 21
` 2.2.5.2 Classless Inter Domain Routing (CIDR) .......... 23
` 2.2.6 IP Multicasting .................................. 24
` 2.2.7 Unnumbered Lines and Networks Prefixes ........... 25
` 2.2.8 Notable Oddities ................................. 26
` 2.2.8.1 Embedded Routers ............................... 26
` 2.2.8.2 Transparent Routers ............................ 27
` 2.3 Router Characteristics ............................. 28
` 2.4 Architectural Assumptions .......................... 31
` 3. LINK LAYER .......................................... 32
` 3.1 INTRODUCTION ....................................... 32
` 3.2 LINK/INTERNET LAYER INTERFACE ...................... 33
` 3.3 SPECIFIC ISSUES .................................... 34
` 3.3.1 Trailer Encapsulation ............................ 34
` 3.3.2 Address Resolution Protocol - ARP ................ 34
` 3.3.3 Ethernet and 802.3 Coexistence ................... 35
` 3.3.4 Maximum Transmission Unit - MTU .................. 35
` 3.3.5 Point-to-Point Protocol - PPP .................... 35
` 3.3.5.1 Introduction ................................... 36
` 3.3.5.2 Link Control Protocol (LCP) Options ............ 36
` 3.3.5.3 IP Control Protocol (IPCP) Options ............. 38
` 3.3.6 Interface Testing ................................ 38
` 4. INTERNET LAYER - PROTOCOLS .......................... 39
` 4.1 INTRODUCTION ....................................... 39
` 4.2 INTERNET PROTOCOL - IP ............................. 39
` 4.2.1 INTRODUCTION ..................................... 39
` 4.2.2 PROTOCOL WALK-THROUGH ............................ 40
` 4.2.2.1 Options: RFC 791 Section 3.2 ................... 40
` 4.2.2.2 Addresses in Options: RFC 791 Section 3.1 ...... 42
` 4.2.2.3 Unused IP Header Bits: RFC 791 Section 3.1 ..... 43
` 4.2.2.4 Type of Service: RFC 791 Section 3.1 ........... 44
` 4.2.2.5 Header Checksum: RFC 791 Section 3.1 ........... 44
` 4.2.2.6 Unrecognized Header Options: RFC 791,
` Section 3.1 .................................... 44
` 4.2.2.7 Fragmentation: RFC 791 Section 3.2 ............. 45
` 4.2.2.8 Reassembly: RFC 791 Section 3.2 ................ 46
` 4.2.2.9 Time to Live: RFC 791 Section 3.2 .............. 46
` 4.2.2.10 Multi-subnet Broadcasts: RFC 922 .............. 47
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` 4.2.2.11 Addressing: RFC 791 Section 3.2 ............... 47
` 4.2.3 SPECIFIC ISSUES .................................. 50
` 4.2.3.1 IP Broadcast Addresses ......................... 50
` 4.2.3.2 IP Multicasting ................................ 50
` 4.2.3.3 Path MTU Discovery ............................. 51
` 4.2.3.4 Subnetting ..................................... 51
` 4.3 INTERNET CONTROL MESSAGE PROTOCOL - ICMP ........... 52
` 4.3.1 INTRODUCTION ..................................... 52
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` 4.3.2 GENERAL ISSUES ................................... 53
` 4.3.2.1 Unknown Message Types .......................... 53
` 4.3.2.2 ICMP Message TTL ............................... 53
` 4.3.2.3 Original Message Header ........................ 53
` 4.3.2.4 ICMP Message Source Address .................... 53
` 4.3.2.5 TOS and Precedence ............................. 54
` 4.3.2.6 Source Route ................................... 54
` 4.3.2.7 When Not to Send ICMP Errors ................... 55
` 4.3.2.8 Rate Limiting .................................. 56
` 4.3.3 SPECIFIC ISSUES .................................. 56
` 4.3.3.1 Destination Unreachable ........................ 56
` 4.3.3.2 Redirect ....................................... 57
` 4.3.3.3 Source Quench .................................. 57
` 4.3.3.4 Time Exceeded .................................. 58
` 4.3.3.5 Parameter Problem .............................. 58
` 4.3.3.6 Echo Request/Reply ............................. 58
` 4.3.3.7 Information Request/Reply ...................... 59
` 4.3.3.8 Timestamp and Timestamp Reply .................. 59
` 4.3.3.9 Address Mask Request/Reply ..................... 61
` 4.3.3.10 Router Advertisement and Solicitations ........ 62
` 4.4 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP .......... 62
` 5. INTERNET LAYER - FORWARDING ......................... 63
` 5.1 INTRODUCTION ....................................... 63
` 5.2 FORWARDING WALK-THROUGH ............................ 63
` 5.2.1 Forwarding Algorithm ............................. 63
` 5.2.1.1 General ........................................ 64
` 5.2.1.2 Unicast ........................................ 64
` 5.2.1.3 Multicast ...................................... 65
` 5.2.2 IP Header Validation ............................. 67
` 5.2.3 Local Delivery Decision .......................... 69
` 5.2.4 Determining the Next Hop Address ................. 71
` 5.2.4.1 IP Destination Address ......................... 72
` 5.2.4.2 Local/Remote Decision .......................... 72
` 5.2.4.3 Next Hop Address ............................... 74
` 5.2.4.4 Administrative Preference ...................... 77
` 5.2.4.5 Load Splitting ................................. 79
` 5.2.5 Unused IP Header Bits: RFC-791 Section 3.1 ....... 79
` 5.2.6 Fragmentation and Reassembly: RFC-791,
` Section 3.2 ...................................... 80
` 5.2.7 Internet Control Message Protocol - ICMP ......... 80
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` 5.2.7.1 Destination Unreachable ........................ 80
` 5.2.7.2 Redirect ....................................... 82
` 5.2.7.3 Time Exceeded .................................. 84
` 5.2.8 INTERNET GROUP MANAGEMENT PROTOCOL - IGMP ........ 84
` 5.3 SPECIFIC ISSUES .................................... 85
` 5.3.1 Time to Live (TTL) ............................... 85
` 5.3.2 Type of Service (TOS) ............................ 86
` 5.3.3 IP Precedence .................................... 87
` 5.3.3.1 Precedence-Ordered Queue Service ............... 88
` 5.3.3.2 Lower Layer Precedence Mappings ................ 89
` 5.3.3.3 Precedence Handling For All Routers ............ 90
` 5.3.4 Forwarding of Link Layer Broadcasts .............. 92
` 5.3.5 Forwarding of Internet Layer Broadcasts .......... 92
` 5.3.5.1 Limited Broadcasts ............................. 93
` 5.3.5.2 Directed Broadcasts ............................ 93
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` 5.3.5.3 All-subnets-directed Broadcasts ................ 94
` 5.3.5.4 Subnet-directed Broadcasts .................... 94
` 5.3.6 Congestion Control ............................... 94
` 5.3.7 Martian Address Filtering ........................ 96
` 5.3.8 Source Address Validation ........................ 97
` 5.3.9 Packet Filtering and Access Lists ................ 97
` 5.3.10 Multicast Routing ............................... 98
` 5.3.11 Controls on Forwarding .......................... 98
` 5.3.12 State Changes ................................... 99
` 5.3.12.1 When a Router Ceases Forwarding ............... 99
` 5.3.12.2 When a Router Starts Forwarding ............... 100
` 5.3.12.3 When an Interface Fails or is Disabled ........ 100
` 5.3.12.4 When an Interface is Enabled .................. 100
` 5.3.13 IP Options ...................................... 101
` 5.3.13.1 Unrecognized Options .......................... 101
` 5.3.13.2 Security Option ............................... 101
` 5.3.13.3 Stream Identifier Option ...................... 101
` 5.3.13.4 Source Route Options .......................... 101
` 5.3.13.5 Record Route Option ........................... 102
` 5.3.13.6 Timestamp Option .............................. 102
` 6. TRANSPORT LAYER ..................................... 103
` 6.1 USER DATAGRAM PROTOCOL - UDP ....................... 103
` 6.2 TRANSMISSION CONTROL PROTOCOL - TCP ................ 104
` 7. APPLICATION LAYER - ROUTING PROTOCOLS ............... 106
` 7.1 INTRODUCTION ....................................... 106
` 7.1.1 Routing Security Considerations .................. 106
` 7.1.2 Precedence ....................................... 107
` 7.1.3 Message Validation ............................... 107
` 7.2 INTERIOR GATEWAY PROTOCOLS ......................... 107
` 7.2.1 INTRODUCTION ..................................... 107
` 7.2.2 OPEN SHORTEST PATH FIRST - OSPF .................. 108
` 7.2.3 INTERMEDIATE SYSTEM TO INTERMEDIATE SYSTEM -
` DUAL IS-IS ....................................... 108
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` 7.3 EXTERIOR GATEWAY PROTOCOLS ........................ 109
` 7.3.1 INTRODUCTION .................................... 109
` 7.3.2 BORDER GATEWAY PROTOCOL - BGP .................... 109
` 7.3.2.1 Introduction ................................... 109
` 7.3.2.2 Protocol Walk-through .......................... 110
` 7.3.3 INTER-AS ROUTING WITHOUT AN EXTERIOR PROTOCOL
` .................................................. 110
` 7.4 STATIC ROUTING ..................................... 111
` 7.5 FILTERING OF ROUTING INFORMATION ................... 112
` 7.5.1 Route Validation ................................. 113
` 7.5.2 Basic Route Filtering ............................ 113
` 7.5.3 Advanced Route Filtering ......................... 114
` 7.6 INTER-ROUTING-PROTOCOL INFORMATION EXCHANGE ........ 114
` 8. APPLICATION LAYER - NETWORK MANAGEMENT PROTOCOLS
` ..................................................... 115
` 8.1 The Simple Network Management Protocol - SNMP ...... 115
` 8.1.1 SNMP Protocol Elements ........................... 115
` 8.2 Community Table .................................... 116
` 8.3 Standard MIBS ...................................... 118
` 8.4 Vendor Specific MIBS ............................... 119
` 8.5 Saving Changes ..................................... 120
` 9. APPLICATION LAYER - MISCELLANEOUS PROTOCOLS ......... 120
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` 9.1 BOOTP .............................................. 120
` 9.1.1 Introduction ..................................... 120
` 9.1.2 BOOTP Relay Agents ............................... 121
` 10. OPERATIONS AND MAINTENANCE ......................... 122
` 10.1 Introduction ...................................... 122
` 10.2 Router Initialization ............................. 123
` 10.2.1 Minimum Router Configuration .................... 123
` 10.2.2 Address and Prefix Initialization ............... 124
` 10.2.3 Network Booting using BOOTP and TFTP ............ 125
` 10.3 Operation and Maintenance ......................... 126
` 10.3.1 Introduction .................................... 126
` 10.3.2 Out Of Band Access .............................. 127
` 10.3.2 Router O&M Functions ............................ 127
` 10.3.2.1 Maintenance - Hardware Diagnosis .............. 127
` 10.3.2.2 Control - Dumping and Rebooting ............... 127
` 10.3.2.3 Control - Configuring the Router .............. 128
` 10.3.2.4 Net Booting of System Software ................ 128
` 10.3.2.5 Detecting and responding to misconfiguration
` ............................................... 129
` 10.3.2.6 Minimizing Disruption ......................... 130
` 10.3.2.7 Control - Troubleshooting Problems ............ 130
` 10.4 Security Considerations ........................... 131
` 10.4.1 Auditing and Audit Trails ....................... 131
` 10.4.2 Configuration Control ........................... 132
` 11. REFERENCES ......................................... 133
` APPENDIX A. REQUIREMENTS FOR SOURCE-ROUTING HOSTS ...... 145
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` APPENDIX B. GLOSSARY ................................... 146
` APPENDIX C. FUTURE DIRECTIONS .......................... 152
` APPENDIX D. Multicast Routing Protocols ................ 154
` D.1 Introduction ....................................... 154
` D.2 Distance Vector Multicast Routing Protocol -
` DVMRP .............................................. 154
` D.3 Multicast Extensions to OSPF - MOSPF ............... 154
` D.4 Protocol Independent Multicast - PIM ............... 155
` APPENDIX E Additional Next-Hop Selection Algorithms
` ................................................... 155
` E.1. Some Historical Perspective ....................... 155
` E.2. Additional Pruning Rules .......................... 157
` E.3 Some Route Lookup Algorithms ....................... 159
` E.3.1 The Revised Classic Algorithm .................... 159
` E.3.2 The Variant Router Requirements Algorithm ........ 160
` E.3.3 The OSPF Algorithm ............................... 160
` E.3.4 The Integrated IS-IS Algorithm ................... 162
` Security Considerations ................................ 163
` APPENDIX F: HISTORICAL ROUTING PROTOCOLS ............... 164
` F.1 EXTERIOR GATEWAY PROTOCOL - EGP .................... 164
` F.1.1 Introduction ..................................... 164
` F.1.2 Protocol Walk-through ............................ 165
` F.2 ROUTING INFORMATION PROTOCOL - RIP ................. 167
` F.2.1 Introduction ..................................... 167
` F.2.2 Protocol Walk-Through ............................ 167
` F.2.3 Specific Issues .................................. 172
` F.3 GATEWAY TO GATEWAY PROTOCOL - GGP .................. 173
` Acknowledgments ........................................ 173
` Editor's Address ....................................... 175
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`1. INTRODUCTION
`
`rfc1812.txt
`
` This memo replaces for RFC 1716, "Requirements for Internet Gateways"
` ([INTRO:1]).
`
` This memo defines and discusses requirements for devices that perform
` the network layer forwarding function of the Internet protocol suite.
` The Internet community usually refers to such devices as IP routers or
` simply routers; The OSI community refers to such devices as
` intermediate systems. Many older Internet documents refer to these
` devices as gateways, a name which more recently has largely passed out
` of favor to avoid confusion with application gateways.
`
` An IP router can be distinguished from other sorts of packet switching
` devices in that a router examines the IP protocol header as part of
` the switching process. It generally removes the Link Layer header a
` message was received with, modifies the IP header, and replaces the
` Link Layer header for retransmission.
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`RFC 1812 Requirements for IP Version 4 Routers June 1995
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` The authors of this memo recognize, as should its readers, that many
` routers support more than one protocol. Support for multiple protocol
` suites will be required in increasingly large parts of the Internet in
` the future. This memo, however, does not attempt to specify Internet
` requirements for protocol suites other than TCP/IP.
`
` This document enumerates standard protocols that a router connected to
` the Internet must use, and it incorporates by reference the RFCs and
` other documents describing the current specifications for these
` protocols. It corrects errors in the referenced documents and adds
` additional discussion and guidance for an implementor.
`
` For each protocol, this memo also contains an explicit set of
` requirements, recommendations, and options. The reader must
` understand that the list of requirements in this memo is incomplete by
` itself. The complete set of requirements for an Internet protocol
` router is primarily defined in the standard protocol specification
` documents, with the corrections, amendments, and supplements contained
` in this memo.
`
` This memo should be read in conjunction with the Requirements for
` Internet Hosts RFCs ([INTRO:2] and [INTRO:3]). Internet hosts and
` routers must both be capable of originating IP datagrams and receiving
` IP datagrams destined for them. The major distinction between
` Internet hosts and routers is that routers implement forwarding
` algorithms, while Internet hosts do not require forwarding
` capabilities. Any Internet host acting as a router must adhere to the
` requirements contained in this memo.
`
` The goal of open system interconnection dictates that routers must
` function correctly as Internet hosts when necessary. To achieve this,
` this memo provides guidelines for such instances. For simplification
` and ease of document updates, this memo tries to avoid overlapping
` discussions of host requirements with [INTRO:2] and [INTRO:3] and
` incorporates the relevant requirements of those documents by
` reference. In some cases the requirements stated in [INTRO:2] and
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` [INTRO:3] are superseded by this document.
`
` A good-faith implementation of the protocols produced after careful
` reading of the RFCs should differ from the requirements of this memo
` in only minor ways. Producing such an implementation often requires
` some interaction with the Internet technical community, and must
` follow good communications software engineering practices. In many
` cases, the requirements in this document are already stated or implied
` in the standard protocol documents, so that their inclusion here is,
` in a sense, redundant. They were included because some past
` implementation has made the wrong choice, causing problems of
` interoperability, performance, and/or robustness.
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`RFC 1812 Requirements for IP Version 4 Routers June 1995
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` This memo includes discussion and explanation of many of the
` requirements and recommendations. A simple list of requirements would
` be dangerous, because:
`
` o Some required features are more important than others, and some
` features are optional.
`
` o Some features are critical in some applications of routers but
` irrelevant in others.
`
` o There may be valid reasons why particular vendor products that are
` designed for restricted contexts might choose to use different
` specifications.
`
` However, the specifications of this memo must be followed to meet the
` general goal of arbitrary router interoperation across the diversity
` and complexity of the Internet. Although most current implementations
` fail to meet these requirements in various ways, some minor and some
` major, this specification is the ideal towards which we need to move.
`
` These requirements are based on the current level of Internet
` architecture. This memo will be updated as required to provide
` additional clarifications or to include additional information in
` those areas in which specifications are still evolving.
`
`1.1 Reading this Document
`
`1.1.1 Organization
`
` This memo emulates the layered organization used by [INTRO:2] and
` [INTRO:3]. Thus, Chapter 2 describes the layers found in the Internet
` architecture. Chapter 3 covers the Link Layer. Chapters 4 and 5 are
` concerned with the Internet Layer protocols and forwarding algorithms.
` Chapter 6 covers the Transport Layer. Upper layer protocols are
` divided among Chapters 7, 8, and 9. Chapter 7 discusses the protocols
` which routers use to exchange routing information with each other.
` Chapter 8 discusses network management. Chapter 9 discusses other
` upper layer protocols. The final chapter covers operations and
` maintenance features. This organization was chosen for simplicity,
` clarity, and consistency with the Host Requirements RFCs. Appendices
` to this memo include a bibliography, a glossary, and some conjectures
` about future directions of router standards.
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` In describing the requirements, we assume that an implementation
` strictly mirrors the layering of the protocols. However, strict
` layering is an imperfect model, both for the protocol suite and for
` recommended implementation approaches. Protocols in different layers
` interact in complex and sometimes subtle ways, and particular
`
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` functions often involve multiple layers. There are many design
` choices in an implementation, many of which involve creative breaking
` of strict layering. Every implementor is urged to read [INTRO:4] and
` [INTRO:5].
`
` Each major section of this memo is organized into the following
` subsections:
`
` (1) Introduction
`
` (2) Protocol Walk-Through - considers the protocol specification
` documents section-by-section, correcting errors, stating
` requirements that may be ambiguous or ill-defined, and providing
` further clarification or explanation.
`
` (3) Specific Issues - discusses protocol design and implementation
` issues that were not included in the walk-through.
`
` Under many of the individual topics in this memo, there is
` parenthetical material labeled DISCUSSION or IMPLEMENTATION. This
` material is intended to give a justification, clarification or
` explanation to the preceding requirements text. The implementation
` material contains suggested approaches that an implementor may want to
` consider. The DISCUSSION and IMPLEMENTATION sections are not part of
` the standard.
`
`1.1.2 Requirements
`
` In this memo, the words that are used to define the significance of
` each particular requirement are capitalized. These words are:
`
` o MUST
` This word means that the item is an absolute requirement of the
` specification. Violation of such a requirement is a fundamental
` error; there is no case where it is justified.
`
` o MUST IMPLEMENT
` This phrase means that this specification requires that the item be
` implemented, but does not require that it be enabled by default.
`
` o MUST NOT
` This phrase means that the item is an absolute prohibition of the
` specification.
`
` o SHOULD
` This word means that there may exist valid reasons in particular
` circumstances to ignore this item, but the full implications should
` be understood and the case carefully weighed before choosing a
`
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` different course.
`
` o SHOULD IMPLEMENT
` This phrase is similar in meaning to SHOULD, but is used when we
` recommend that a particular feature be provided but does not
` necessarily recommend that it be enabled by default.
`
` o SHOULD NOT
` This phrase means that there may exist valid reasons in particular
` circumstances when the described behavior is acceptable or even
` useful. Even so, the full implications should be understood and
` the case carefully weighed before implementing any behavior
` described with this label.
`
` o MAY
` This word means that this item is truly optional. One vendor may
` choose to include the item because a particular marketplace
` requires it or because it enhances the product, for example;
` another vendor may omit the same item.
`
`1.1.3 Compliance
`
` Some requirements are applicable to all routers. Other requirements
` are applicable only to those which implement particular features or
` protocols. In the following paragraphs, relevant refers to the union
` of the requirements applicable to all routers and the set of
` requirements applicable to a particular router because of the set of
` features and protocols it has implemented.
`
` Note that not all Relevant requirements are stated directly in this
` memo. Various parts of this memo incorporate by reference sections of
` the Host Requirements specification, [INTRO:2] and [INTRO:3]. For
` purposes of determining compliance with this memo, it does not matter
` whether a Relevant requirement is stated directly in this memo or
` merely incorporated by reference from one of those documents.
`
` An implementation is said to be conditionally compliant if it
` satisfies all the Relevant MUST, MUST IMPLEMENT, and MUST NOT
` requirements. An implementation is said to be unconditionally
` compliant if it is conditionally compliant and also satisfies all the
` Relevant SHOULD, SHOULD IMPLEMENT, and SHOULD NOT requirements. An
` implementation is not compliant if it is not conditionally compliant
` (i.e., it fails to satisfy one or more of the Relevant MUST, MUST
` IMPLEMENT, or MUST NOT requirements).
`
` This specification occasionally indicates that an implementation
` SHOULD implement a management variable, and that it SHOULD have a
` certain default value. An unconditionally compliant implementation
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` implements the default behavior, and if there are other implemented
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` behaviors implements the variable. A conditionally compliant
` implementation clearly documents what the default setting of the
` variable is or, in the absence of the implementation of a variable,
` may be construed to be. An implementation that both fails to
` implement the variable and chooses a different behavior is not
` compliant.
`
` For any of the SHOULD and SHOULD NOT requirements, a router may
` provide a configuration option that will cause the router to act other
` than as specified by the requirement. Having such a configuration
` option does not void a router's claim to unconditional compliance if
` the option has a default setting, and that setting causes the router
` to operate in the required manner.
`
` Likewise, routers may provide, except where explicitly prohibited by
` this memo, options which cause them to violate MUST or MUST NOT
` requirements. A router that provides such options is compliant
` (either fully or conditionally) if and only if each such option has a
` default setting that causes the router to conform to the requirements
` of this memo. Please note that the authors of this memo, although
` aware of market realities, strongly recommend against provision of
` such options. Requirements are labeled MUST or MUST NOT because
` experts in the field have judged them to be particularly important to
` interoperability or proper functioning in the Internet. Vendors
` should weigh carefully the customer support costs of providing options
` that violate those rules.
`
` Of course, this memo is not a complete specification of an IP router,
` but rather is closer to what in the OSI world is called a profile.
` For example, this memo requires that a number of protocols be
` implemented. Although most of the contents of their protocol
` specifications are not repeated in this memo, implementors are
` nonetheless required to implement the protocols according to those
` specifications.
`
`1.2 Relationships to Other Standards
`
` There are several reference documents of interest in checking the
` status of protocol specifications and standardization:
`
` o INTERNET OFFICIAL PROTOCOL STANDARDS
` This document describes the Internet standards process and lists
` the standards status of the protocols. As of this writing, the
` current version of this document is STD 1, RFC 1780, [ARCH:7].
` This document is periodically re-issued. You should always
` consult an RFC repository and use the latest version of this
` document.
`
`Baker Standards Track [Page 11]
`

`RFC 1812 Requirements for IP Version 4 Routers June 1995
`
` o Assigned Numbers
` This document lists the assigned values of the parameters used in
` the various protocols. For example, it lists IP protocol codes,
` TCP port numbers, Telnet Option Codes, ARP hardware types, and
` Terminal Type names. As of this writing, the current version of
` this document is STD 2, RFC 1700, [INTRO:7]. This document is
` periodically re-issued. You should always consult an RFC
` repository and use the latest version of this document.
`Page 10
`
`

`
`rfc1812.txt
`
` o Host Requirements
` This pair of documents reviews the specifications that apply to
` hosts and supplies guidance and clarification for any
` ambiguities. Note that these requirements also apply to routers,
` except where otherwise specified in this memo. As of this
` writing, the current versions of these documents are RFC 1122 and
` RFC 1123 (STD 3), [INTRO:2] and [INTRO:3].
`
` o Router Requirements (formerly Gateway Requirements)
` This memo.
`
` Note that these documents are revised and updated at different times;
` in case of differences between these documents, the most recent must
` prevail.
`
` These and other Internet protocol document